Recent advancements of two-dimensional transition metal dichalcogenides and their applications in electrocatalysis and energy storage

The discovery of graphene has stirred an intensive research interest in two-dimensional (2D) materials, but its lack of an electronic band gap has stimulated the research for novel materials with semiconducting character. The past few years have witnessed an impressive advancement in 2D materials from fundamental studies to the development of next generation of technologies and materials engineering. Among them, 2D transition metal dichalcogenides (TMDs) have been extensively studied in various areas of research since last few decades, and these 2D TMDs are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se, or Te), furnish an auspicious alternative. Due to its unique physical and chemical properties, 2D monolayer TMDs exhibit a distinctive combination of atomic-scale thickness, direct band gap, strong spin–orbit coupling, and favorable electronic and mechanical properties. These properties make the 2D TMD materials (such as MoS2, MoSe2, WTe2, WS2, and WSe2) more interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, electrocatalysis, energy harvesting, flexible electronics, water splitting, DNA sequencing, and personalized medicine. They exhibit tunable electronic band gaps that can undergo a transition from an indirect band gap (bulk crystal structure) to a direct band gap (2D monolayer nanosheets, i.e., slab structure). Because of its robustness, 2D monolayer MoS2 is the most studied material in this family and especially for the applications of electrocatalysis, H2 evolution reactions (HER), etc. Current state-of-the-art catalysts still rely on expensive and rare noble metals; however, the relatively cheap and abundant TMDs have emerged as exceptionally promising alternative electrocatalysts for HER. In this review, we focus on the development of 2D TMDs, their synthesis methods, electronic structures and phases of the TMDs, theoretical modelling of the 2D TMDs, computations of electronic properties, and their potential applications in HER. They have been widely considered potential candidates for HER electrocatalysts because of their low cost, good electrochemical stability in acidic conditions, and its nearly thermoneutral hydrogen adsorption energy. The mechanism of hydrogen adsorption on TMDs plays an important role in optimizing HER activity. This review emphasizes on recent progress in improving the catalytic properties of TMDs toward highly efficient production of H2 by electrochemical HER. Combining theoretical and experimental considerations, a summary of the progress to date is provided, and a pathway forward for viable hydrogen evolution from TMD driven catalysis is concluded.